Protein targeting involves transporting proteins to their proper destinations after synthesis so they can perform their functions. There are two main pathways: co-translational targeting transports proteins during translation to the ER, Golgi and secretory pathway, while post-translational targeting transports proteins after translation to the nucleus, mitochondria and peroxisomes. Targeting sequences on the protein interact with receptors to mediate transport through membrane channels using energy from GTP or ATP hydrolysis. Defects in protein targeting can cause diseases like Zellweger syndrome, primary hyperoxaluria and cystic fibrosis.

1. Protein Targeting
M.Prasad Naidu
MSc Medical Biochemistry,
Ph.D.Research Scholar

2. Each eukaryotic cell is
subdivided into
functionally distinct,
membrane-bound
compartments –
organelles
 Each compartment has
its own distinct set of
proteins = functions
 A complex distribution
system moves proteins
from the place of
synthesis to its proper
destination

3. Protein targeting
 Protein has to be correctly localized to perform proper
function.
 Receptors – plasma membrane
 DNA polymerase – nucleus
 Catalase – peroxisomes
 Insulin – outside

4.  All proteins begin to be synthesized on cytosolic
ribosomes.
 Sorting or translocation can occur
 Co-translational
 Post-translational
 If the protein is for cytosolic functins, the synthesis
will be finished on free ribosomes and the peptide is
released into the cytosol.

5.  If the protein is destined for nucleus,
mitochondria or peroxisomes the synthesis is
also finished on cytoplasmic ribosomes and the
peptide is released to the cytosol (to be sorted later or
post-translationally).
 If the protein is going to be secreted from the cell or it
destined for the membranes the ribosome with the
nascent peptide is targeted to the ER (ER becomes
rough) and sorting is done during translation (co-
translationally).

6.  post-translational
targeting:
•nucleus
•mitochondria
•Peroxisomes
 co-translational
targeting
(secretory pathway):
•ER
•Golgi
•lysosomes
•plasma membrane
•secreted proteins

8. Targeting sequence
 Characteristic for the destination not the protein
 Part of the polypeptide
 Can be cleaved later by signal peptidase or remain
permanent part of protein
 Can be located on N-, C-terminus or in the middle of
the protein

9. ( GUENTER BLOBEL – NP –
1999 )

10. Transport of the new protein
into
mitochondria
 Most mitochondrial proteins are encoded by nuclear
DNA
 Only very few are encoded by mitochondrial DNA and
synthesized on mitochondrial ribosomes

11. Mitochondrial targeting signals
 Usually located at N-terminus of precursor
polypeptide
 Usually removed in mitochondrial matrix

12. Receptor/translocation channels
in
mitochondria
 Tom – translocase of the outer mitochondrial
membrane
 Tim - translocase of the inner mitochondrial
membrane

13.  Mitochondrial proteins are synthesized in cytosol as
precursors
 Bind to cytosolic chaperones(Hsp 70) to keep them
unfolded until they ready to be translocated
 Energy from ATP

15.  Some outer membrane proteins insert themselves in
the membrane while in transit
 Intermembrane space proteins remain there and fold
 Protein destined to matrix passes through Tom 40 and
then Tim (inner membrane translocon)

16. Peroxisomes
 Single membrane organelle
 Matrix contains oxidative enzymes
 Lipid oxidation without ATP production
 Proteins encoded by nuclear DNA (all have to be
imported).

17. Transport into peroxisomes
 Proteins are synthesized and fully folded in cytosol
 Fully functional, fully folded protein is transported!
 Import requires ATP hydrolysis
 Peroxisome targeting sequences
 PTS1 on C-terminus, very conserved
 PTS2 on N-terminus, just few proteins

18.  Peroxins - peroxisome transport receptors
 Bind to proteins with PTS1 and dock to the
translocation channel
 The complex is transported through the
membrane
 Protein is released
 Peroxin is recycled

20. Transport into the nucleus
 All proteins found in the nucleus are synthesized in
the cytoplasm
 Examples:
 Histones
 Ribosomal proteins
 DNA and RNA polymerases
 Transcription factors

21.  Transport requires nuclear localization sequences
(NLS)
 Transport occurs through the nuclear pores
 Nuclear import receptor (Importin α and β)
 Energy from GTP
 GTPase Ran
 Fully folded proteins are transported

22.  Importin α and β bind to the protein to be transported
 Nuclear localization signal binds to importin α
 The complex is translocated through the nuclear
membrane

23.  Activated Ran (GTP) causes the complex to dissociate
 Ran transports importin β back to cytosol
 Importin α becomes a part of export receptor

25. and their co-translational
translocation across the ER
membrane
 ER signal sequence emerges
 The binding by a signal-recongition particles
(SRP)
 SRP delivers the ribosome/nascent polypeptide
complex to the SRP receptor in the ER
membrane, and GTP binding

27. synthesized on
the rough ER

29. Diseases due to defective
protein targeting
1. ZELLWEGER SYNDROME
2. PRIMARY HYPEROXALURIA
3. FAMILIAL HYPERCHOLESTROLEMIA
4. CYSTIC FIBROSIS
5. INCLUSION CELL DISEASE

30. Thank you